1. Field of Invention
The present invention relates to an engine oil composition for automobiles. More particularly, the invention relate to the long life and fuel-saving engine oil composition which can suppress the friction loss in the engine to a low level for a long time.
2. Description of Related Art
With the progress of the engines, the automobile engine oil compositions (hereinafter referred to briefly as "engine oil compositions") have been required to possess various performances such as wear resistance, oxidation stability, and detergent dispersibility. Recently, in order to suppress the earth from getting warmer due to increase in the content of CO.sub.2 in the atmosphere, how to improve the mileage of the automobiles is an important problem. Accordingly, the fuel saving has been also strongly required with respect to the engine oils.
Ordinarily, the engine oil composition is composed of a mixture of a base oil purified from petroleum, added with additives such as detergent, an antioxidant, an anti-wear agent, and a viscosity index improver. In order to increase the fuel efficiency (mileage) of the engine oil, for example, the viscosity of the engine oil is lowered by decreasing the viscosity of the base oil or changing the viscosity index improver. However, friction cannot be reduced in the case of the above ordinary engine oil composition in such an area as a boundary lubricating condition where the viscosity does not contributes to mitigation of the friction. Consequently, a friction modifier (FM) has recently come to be added so as to reduce the wearing in the boundary lubricating area. With respect to the friction modifiers, it is known that organic molybdenum compound such as molybdenum dithiocarbamate (MoDTC) and oxymolybdenum organo phosphodithioate sulfide (MoDTP) are highly effective as described in JP-B 3-23595.
However, as the time passes, each of the above organic molybdenum compounds used in the engine oil composition is consumed. Therefore, though the fresh engine oil composition gives a low fuel consumption rate, such a low fuel consumption rate of the engine oil composition is deteriorated with the lapse of time. In order to lessen the above drawback, it may be considered that the addition amount of the organic molybdenum compound in a fresh oil is increased. However, if the addition amount of the organic molybdenum compound is merely increased, the cost of the product becomes higher, which is economically unfavorable. Further, among the organic molybdenum compounds, MoDTP contains phosphorus, so that a phosphorus compound may deposit on the surface of an exhaust gas catalyst to deteriorate the catalytic activity. Therefore, the addition amount of the MoDTP cannot be increased beyond a given level.
On the other hand, since MoDTC contains no phosphorus, increase in its addition amount does not cause decrease in the catalytic activity. However, since MoDTC has a small friction-mitigating effect, it may be considered that MoDTC is used in combination with zinc dithiophosphate (ZnDTP) as an anti-wear agent so as to supplement the wear-mitigating effect of the former. ZnDTP has been frequently used, as antioxidant and antiwear agent, in the engine oil compositions. However, since ZnDTP contains phosphorus and gives adverse influence upon the exhaust gas catalyst as mentioned above, its addition amount is limited so that good friction-mitigating effect cannot unfavorably be maintained for a long time. Further, it is proposed that MoDTC be used in combination with a sulfur-based extreme pressure additive (See JP-B 5-83599). This combination does not afford adverse effect upon the exhaust gas catalyst, but it encounters a practically great problem upon the engine oil composition in that wear largely occurs in the valve train system.